1. Field of the Invention
This invention relates to stretched films free from the problem of paper powder, and having excellent printing and writing properties and excellent strength.
2. Description of the Prior Art
It has heretofore been known that an opaque paper having writing and printing properties can be produced by compounding a polyolefin resin or a mixture of a polyolefin resin and a polystyrene resin with an inorganic fine powder, processing the resulting mixture to form a film, and then stretching the film in at least one direction (see Japanese Patent Publication Nos. 40794/1971, 3902/1972 and 9229/1979, U.S. Pat. No. 3,154,461, and British Pat. Nos. 1,096,064 and 1,090,059).
In such methods, however, inorganic powders such as calcium carbonate, clay, diatomaceous earth, titanium oxide, etc. are used to provide printing properties, and these inorganic powders give rise to various printing problems. For example, inorganic fillers or powders projecting from the surface of the stretched film fall off and mix with an offset printing ink, deteriorating the ink, shortening the durability of the ink and wearing away an aluminum plate (this is the so-called "paper powder" problem), and they stick together to form big particles on which no gravure printing ink can be printed, leading to the occurrence of the phenomenon "whiteness" (i.e., the fillers on which the printing ink is adhered separate from the stretched film and the omitted portions become white).
Moreover, since such inorganic fine powders act to lower the transparency of film, if the amount of the inorganic fine powder added is reduced to obtain translucent synthetic papers such as tracing paper, paper for a poster to be used on an electric sign, etc., offset printing and writing properties will be reduced. Therefore, it is difficult to obtain synthetic papers having a good balance between transparency and writing or offset printing properties.
These tendencies are more marked when a mixture of polyolefin and polystyrene is used as the film.
Furthermore, since a transparent stretched polypropylene film is poor in affinity to printing ink for cellulose based films in which a cellulose based resin is used as a binder (even when a corona discharge treatment is applied onto the surface of the polypropylene film), the printing ink used for propropylene sheets or films is a special ink prepared by using a polyamide-based or polyvinyl chloride-based resin as a binder and diluting the binder with a solvent composed mainly of toluene; such ink low drying properties.
The toluene solvent, however, hardly dissipates from the binder and polypropylene film. Therefore, a long period of time is required for drying after printing. Furthermore, the film is typically not sufficiently dried, and the solvent odor remains in the film, preventing the polypropylene film from applications in the field of food wrapping.
In order to permit the gravure printing of the polypropylene film by use of the ink for cellulose based films, it has been considered necessary to use a mixture of polypropylene and polystyrene. However, a film produced from such a mixture is translucent or opaque, and a transparent wrapping film has not been obtained using such a mixture.